Method of electrolytically etching aluminum



May 3, 1966 Filed Sept. 1

w; POST ETAL 3,249,523

METHOD OF ELECTROLYTICALLY ETCHING ALUMINUM 1, 1962 2 Sheets-Sheet 1 t INVENTORS.

BY MiZw/ie/ ATTORNE Y5.

May 3, 1966 w. POST ET AL v 3,249,523

METHOD OF ELECTROLYTICALLY ETCHING ALUMINUM Filed Sept. 11, 1962 2 Sheets-Sheet 2 /6 T 1 E i l i fi': /Z -55; 36 fi lig IN VENTORS ATTORNE Y5.

United States Patent 3,249,523 METHOD OF ELECTROLYTICALLY ETCHING ALUMINUM Wolfgang Post and Eduard Worschischek, Singen am Hohentwiel, Germany, assignors to Swiss Aluminium Ltd., a joint-stock company of Switzerland Filed Sept. 11, 1962, Ser. No. 222,906 Claims priority, application Switzerland, Sept. 15, 1961,

7 Claims. (Cl. 204-141) The present invention relates to a method of electrolytically etching aluminum surfaces, especially of aluminum electrodes for electrolytic capacitors.

The etching of aluminum electrode foils increases their surface areas, and consequently increases the capacitance of the capacitor, in which these foils are employed. Electrolytic processes are known for etching by using a pulsating direct current, whereby greater increase in surface area is attained compared with a process employing direct current of constant value. By pulsating direct current is meant a uni-directional uninterrupted current Whose magnitudefiuctuates periodically. According to British Patent No. 715,525, the current-time curve of the pulsating current may be of different square or sine wave form, but the degree of pulsations must be within the range of 50 to 250%, i.e. the total current variation expressed as a percentage of the average current value. v

The pulsating current may be generated in different ways, but an unsmoothed rectified current, which could be obtained from 1- or 3-phase alternating current of sine Wave form is said to be unsuitable, as the degree of pulsation is too low.

Besides the degree of pulsation, other etching condi tions, such as the temperature, the current density and the electrolyte concentration also influence the extent of surface increase. In the aforesaid British patent, a high current density, as for example, 50 to 60 amps./dm. and an electrolyte concentration as low as possible, as for example, between 0.1 and 0.2 N are said to be optimum. As electrolyte, aqueous solutions of several acids and salts, especially halogen compounds are employed.

It has been found in accordance with the present invention, that by employing a specific combination of different etching conditions, the surface area may be increased still more. In accordance with the present invention,

the etching is carried out using direct current with a pulsation degree of below 50%, but of at least 5%, an average current density of over 50 amps./dm. an electrolyte concentration in the range of 3 moles per liter up to saturation and a temperature between 70 C. and the boiling point. With these etching conditions, the highest increase in surface area is attained, especially on aluminum electrodes for low voltage capacitors, that means on electrodes which are provided with the dielectric layer by anodic oxidation at a voltage below 100 volts.

In accordance with the present invention, the specified etching conditions must be applied in combination, in order to obtain the highest increase in surface area. If even one of these conditions is changed, as for example, by employing a degree of pulsation of over 50% or an average current density below 50 amps./dm. the surface increase is less. However, the frequency of pulsation may be varied within a wide range, for example, between 50 and 2000 cycles per second or even more. Alterations of the frequency beyond 200 cycles per second have hardly an influence on the extent of surface increase.

The low degree of pulsation employed compared with the known method has the advantage of permitting the use of an unsmoothed, rectified current from 1- or 3-phase alternating current of sine form, which is easily generated. However, as far as certain aspects of the inven- 3,249,523 Patented May 3, 1966 ice tion are concerned, the time-current curve of the pulsating current may also have other forms.

Suitable electrolytes which may be employed in accordance with the present invention are aqueous solutions of chlorides, bromides, iodides and chlorates of the alkaline metals, and especially a sodium chloride solution. The concentration of these compounds-in aqueous solution must be 3 moles per liter or higher. A saturated sodium chloride solution has a concentration of about 5 moles perliter depending somewhat on the temperature. An addition of 1 to 5% by Weight of hydrochloric acid proved to be advantageous. However, the addition should not be too great in order that the aluminum is not attacked by the electrolyte before the current is applied.

The current density is limited by the highest possible electrical load at which the foil becomes overheated and begins to melt.

Various other features of the invention are apparent from the following examples, illustrating the principles of the invention and by the accompanying drawings, in which:

FIGS. 1-3 show the time-current curves employed in connection with the following examples respectively; and

FIGS. 4-6 show the corresponding circuits for generating the pulsating currents employed in connection with the following examples. In these drawings, (1) designates the aluminum foil to be etched, (2) the etching electrolyte solution and (3) the electrolyte container constituting the cathode.

I Example 1 An aluminum foil having a purity of 99.99% Was electrolytically etched during seconds after degreasing and pickling, by feeding continuously through an aqueous solution of 5 moles of sodium chloride per liter (29%) at a temperature of 80 C. The direct pulsating current employed was an unsmoothed rectified current derived from a 3 -phase alternating sine wave current. This pulsating current had an average current density of 80 amps./dm. and a time-current curve as shown in FIG. 1. The fluctuations of the current were of such magnitude as to give a degree of pulsation of 20%. For generating this current, a circuit with a 3-phase rectifier 4, as shown in FIG. 4 was employed.

The increase in surface area obtained by etching the aluminum foil in accordance with Example 1 was measured as a ratio of the capacity values of an etched foiland an unetched foil after formation of the dielectric layer by anodic oxidation at 6 volts in an electrolyte containing boric acid. The increase was 45 to 50 fold.

Example 2 An aluminum foil of 99.85% purity was etched in an aqueous solution containing 5 moles of sodium chloride and 5% by weight of hydrochloric acid at a temperature of C. The etching current Was derived from a direct current generator smoothed land superposed with alternating current in such a way as to yield a pulsating direct current having a degree of pulsation of 20%. The timecurrent curve of this pulsating direct current is shown in FIG. 2 and the circuit is shown in FIG. 5. This circuit comprises a first loop consisting of a direct current generator 5 and a choke 6 in series with the generator for cutting up the alternating current, and a second loop in parallel with said first loop and consisting of an alternating current generator 7 and a condenser 8 in series with said alternating current generator for avoiding direct current flux through said alternating current generator, the two loops being connected in parallel with the aluminum foil 1 and the cathode container 3. The average current density was amps./dm. After 75 seconds of etching, the increase in surface area Was about 40 fold.

J Example 3 Two aluminum foils 1a and 1b of 99.5% purity were etched simultaneously in two aqueous etching solutions 2a and 2b containing 5 moles of sodium chloride per liter at the boiling point. These foils 1a and 1b and cathode containers 3a and 312 for the baths 2a and 2b respectively were connected into an electric circuit shown in FIG. 6. By means of a throw-over switch 9 in this circuit, direct current was fed alternately to both baths 2a and 2b, thereby supplying said baths with interrupted direct current. Resistances 10a and 10b shunted across the switch 9 permittcd additional direct current to flow continuously through the baths. The result was a pulsating direct current' flow having the square wave characteristics shown in FIG. 3 and having a degree of pulsation of The average current density was 100 amps./dm. After 100 seconds of etching, the increase in surface area was 40-45 fold.

If an aluminum foil is etched with a rectified alternating current having a degree of pulsation of over the other conditions being the same as in the specific examples described, there is obtained only a 30 fold increase in surface area. Also, when using lower current densities or electrolyte concentrations, the increase in surface area obtained is less than that obtained by using the conditions in accordance with the present invention.

While the invention has been described with particular reference to specific embodiments, it is to be understood that it is not to be limited thereto but is to be construed broadly and restricted solely by the scope of the appended claims.

What is claimed is:

1. The method of etching aluminum to increase its surface area for use in electrolytic capacitors after a dielectric layer has been anodically formed thereon at a voltage below 100 volts, which comprises subjecting said aluminum to the action of an aqueous electrolytic solution containing essentially a halogen compound of the class consisting of alkali metal chlorides, bromides, iodides and chlorates, by a pulsating direct current having a degree of pulsation of less than 50%, but of at least 5% and an average current density of more than 50 amps./ dm. the concentration of the halogen compound in said solution being 3 moles per liter up to saturation and the temperature of said solution being between C. and the boiling point of said solution.

2. The method as described in claim 1, in which the electrolyte is an aqueous solution of 3 to 5 moles of sodium chloride per liter with the addition of 1 to 5% by weight of hydrochloric acid.

3. The method according to claim 1, in which the pulsating direct current is an unsmoothed rectified current derived from 3-phase alternating current of sine wave form.

4. The method as described in claim 1, in which the pulsating direct current is a smoothed wave current derived from direct current of uniform value modified by alternating current superposed thereon.

'5. The method as described in claim 1, wherein direct current of constant value is continuously supplied to the electrolyte and at the same time, direct current of constant value intermittently interrupted is supplied to the electrolyte to produce the pulsating direct current for said electrolyte.

6. The method as described in claim 5, wherein two etching operations are carried out at the same time in two electrolytic baths, by alternately delivering direct current of constant value to one bath and interrupting the latter current to the other bath, while simultaneously delivering current of constant value to both baths.

7. The method as described in claim 1, wherein the aluminum etched is a foil for forming an electrode for an electrolytic capacitor.

References Cited by the Examiner UNITED STATES PATENTS 2,699,382 1/ 1955 Altenpohl 204-141 7 2,755,237 7/1956 Turner 204-l4l 2,755,238 7/1956 Turner 204-l4l 2,930,741 3/1960 Burger et al. 204-141 3,085,950 4/1963 Thomas et al. 204-141 FOREIGN PATENTS 914,836 7/1963 Great Britain.

WINSTON A. DOUGLAS, Primary Examiner. JOHN H. MACK, Examiner.

R. L. GOOCH, R. K. MIHALEK, Assistant Examiners. 

1. THE METHOD OF ETCHING ALUMINUM TO INCREASE ITS SURFACE AREA FOR USE IN ELECTROLYTIC CAPACITOR AFTER A DIELECTRIC LAYER HAS BEEN ANODICALLY FORMED THEREON AT A VOLTAGE BELOW 100 VOLTS, WHICH COMPRISES SUBJECTING SAID ALUMINUM TO THE ACTION OF AN AQUEOUS ELECTROLYTIC SOLUTION CONTAINING ESSENTIALLY A HALOGEN COMPOUND OF THE CLASS CONSISTING OF ALKALI METAL CHLORIDES, BROMIDES, IODIDES AND CHLORATES, BY A PULSATING DIRECT CURRENT HAVING A DEGREE OF PULSATION OF LESS THAN 50%, BUT OF AT LEAST 5% AND AN AVERAGE CURRENT DENSITY OF MORE THAN 50 AMPS./ DM2, THE CONCENTRATION OF THE HALOGEN COMPOUND IN SAID SOLUTION BEING 3 MOLES PER LITER UP TO SATURATION AND THE TEMPERATURE OF SAID SOLUTION BEING BETWEEN 70*C. AND THE BOILING POINT OF SAID SOLUTION. 